The identification and quantification of analytes (e.g., metabolites) typically requires access to large and expensive instrumentation, such as NMR and mass spectrometers.
Some molecules (like glucose) can be detected via enzymatic assays while others, like vitamin D or certain pesticides, can be detected through immunological methods using chemo-conjugated proteins. However, these low-cost assays are very compound specific, and can take years of trial-and-error to develop.
Metabolomics is a relatively new branch of 'omics science that involves the comprehensive characterization of large numbers of metabolites in cells, tissues and biofluids. Over the past 10 years, interest in metabolomics has grown tremendously. This is because it offers biologists and other life scientists a rapid and effective means to chemically phenotype organisms. It also offers physicians and clinicians a very quick and efficient route to discover or test for disease biomarkers.1-4 Most metabolomic assays involve the use of classical separation technologies such as liquid chromatography or gas chromatography. These are normally coupled with high-end chemical detection instruments such as NMR or mass spectrometers to identify and quantify metabolites from biological samples. The size, sophistication and expense of most metabolomics instruments means that many metabolomics activities must be conducted in well-equipped, multi-million dollar core facilities. The requirement for expensive instruments and highly trained personnel has made metabolomics increasingly inaccessible to many life scientists. Ideally what is missing is a novel approach that “democratizes” metabolomics by making metabolomics assays portable, simple and inexpensive. One way of doing this is to use the power of enzyme or protein-based assays to detect metabolites. Perhaps one of the best known examples of a portable enzyme-based metabolite assay is the blood glucose sensor.5,6 This assay, which uses glucose oxidase as a dual sensor/detector is used by millions of diabetics world-wide.
However, very few enzyme/metabolite systems exist that can generate easy-to-detect signals. Indeed, most of these enzyme/metabolite detection systems require an enzyme that uses a colorimetric cofactor (NADP for instance) or a detectable redox couple (glucose oxidase for instance). Only a small number of enzymatic reactions, including glucose oxidation via glucose oxidase, fit these requirements.7 
The preparation of small molecule-protein conjugates is difficult. Often, chemically modified versions of the analyte must be prepared and various trial-and-error attempts must be performed to develop an optimal analyte analog and to chemically couple that analyte to the protein of choice.14 Difficulties often arise due to inconsistent orientation, placement and abundance of the analyte on the protein surface. Once coupled, an appropriate antibody to the protein bound molecule must be developed; small molecules alone cannot be used to raise antibodies. The resulting protein-hapten antibodies may or may not recognize the pure small molecule, and for the case of metabolites, autoimmunity would pose a problem, leading to more trial and error attempts to create optimal antibodies that recognize both forms. As a result, years of effort must be devoted to generate a useful small-molecule analyte immunoassay. These compound-specific challenges along with the difficulty in preparing appropriate protein conjugates, appropriate colorimetric amplification techniques and appropriate small molecule antibodies have led to a rather modest number of small molecule immunoassays being developed. Only a few different small molecule analyte assays that have been described in the literature or that have reached the market.15,16 This is in contrast to the thousands of protein-based ELISAs that have already been described or developed. Clearly, if small molecule immunoassays are ever to have an impact on metabolomics—where dozens to hundreds of molecules need to be detected—there will need to be a significant improvement in their rate or ease of development.
Presently, there is a lack of a general method that would both simplify the process and shorten the assay development time for small molecule detection.